This invention relates to an acoustic borehole transmitter for generating acoustic energy waves covering the entire acoustic band.
In acoustic borehole logging, an acoustic energy transmitter and one or more receivers disposed at spaced-apart distances from the transmitter are included in a borehole logging tool that is moved through a borehole. Acoustic energy waves generated by the transmitter travel through the subsurface formations surrounding the borehole and are detected by the receiver or receivers. One such logging tool is described in U.S. Pat. Nos. 4,383,308 and 4,432,077 to R. L. Caldwell.
A particularly suitable transmitter for generating low frequency acoustic energy waves in subsurface formations surrounding a borehole employs a Helmholtz resonator device. Helmholtz resonator principles are well known for use in the generation of sound as described in Fundamentals of Acoustics, by L. E. Kinsler, and A. R. Frey, Wiley and Sons, N.Y. (1962), pg. 186. All such resonators are characterized by a closed shell of spherical, cylindrical or other nature containing one or more small apertures. These apertures may be holes in the shell wall or tubes inserted through the shell wall. Neither the configuration of the shell nor the shape of the aperture are critical to the characteristics of the resonator. Such Helmholtz resonators have two fundamental resonances. One is the Helmholtz resonance associated with fluid flow through the aperture. The other is the shell resonance associated with the motion of the shell wall. Briefly, the Helmholtz resonant frequency f is given by: ##EQU1## where
V=volume of shell
L=effective length of aperture (i.e., L.about.l+0.6a),
a=diameter of aperture, and
c=velocity of sound.
Further, the shell resonant frequency is given by: ##EQU2## where,
R=mean radius of shell,
W=shell thickness,
G=shear modulus,
M=mass of shell per unit area, and
v=Poisson ratio of shell material.
Use of such a resonator as an acoustic transmitter in an acoustic borehole logging tool is described in U.S. Pat. No. 4,674,067 to J. Zemanek. However, such an acoustic transmitter is limited in that the diameter of the resonator shell must be no more than 3 to 4 inches for use in borehole logging. Consequently, the acoustic energy output level is limited and may not produce acoustic energy waves of sufficient amplitude to be adequately detected at receiver spaacings greater than 10 to 20 feet. Further, such an acoustic transmitter is limited in frequency response to a fundamental Helmholtz resonance in the range of 0.2 to 2 kilohertz and a fundamental shell resonance in the range of 10 to 20 kilohertz. With such frequency limitation, the transmitter cannot be used in frequency sweep applications over the entire 0 to 20 kilohertz frequency band such as described in U.S. Pat. No. 4,718,046 to Modlin.
It is therefore a specific object of the present invention to provide a Helmholtz resonator with large acoustic energy output and broadband frequency characteristics for use in borehole logging operations wherein long receiver spacings (i.e. greater than 20 ft.) are utilized and wherein sweep frequency applications are required for acoustic tube wave logging of wellbores.